The invention concerns wiring and winding configurations for electric motors and particularly arrangements for small series wound or universal motors of the type typically used in power tools and appliances.
Compactness and low manufacturing costs are generally desirable goals in electric motor design and especially in motors for the highly competitive portable power tool and appliance markets. There, production quantities are often large enough to make automated assembly feasible and advantageous. However, conventional coil winding configurations and termination arrangements still require significant amounts of hand work which inhibit full automation. This residual hand work may include fishing through or threading through of jumper wires or leads from end-to-end or side-to-side of the fields and making their connections, as well as hand insertion of the field coils themselves.
Further, in more complex motors having secondary or auxiliary windings providing an auxiliary function such as dynamic braking, speed change, or reversing the number of connections required to be made to field coil terminations is multiplied and, given the limited amount of space available at the end of the field, it becomes less feasible to provide sufficient space between the terminals to facilitate automated connection.
Electro-dynamic braking systems (taken as an exemplary auxiliary function), with or without specific windings, in small portable tools and appliances are already known. In some tools, such as circular saws, normal unbraked stopping time may be inconveniently long due to the inertia of the motor armature and functional elements such as the circular saw blade. In typical self-excited dynamic braking systems, release of an "on" switch or trigger simultaneously or sequentially interrupts the flow of electrical power to the tool and effects a reverse of polarity in the windings so that, with the aid of the residual magnetic field in the stator, a self-excited electromagnetic field of force is generated, opposing continued rotation of the motor armature in the original running direction, so that rotation is arrested more quickly than by normal windage and friction.
Available conventional dynamic braking systems, with or without a secondary winding, suffer from some shortcomings. In a known circular saw application for example, braking is effected simply, using suitable switching, by reversing the polarity of the saw motor's run winding. But this may result in undesirably rapid deceleration and also high currents and severe arcing, shortening brush, commutator and motor life.
When a secondary winding is provided, specifically for the braking function, winding design may be chosen to moderate the braking action to suit the particular application. However, in known applications especially when the brake field coils are wound opposite hand to the run field coils, the arrangement becomes bulky, making in situ winding less practical and typically requiring relatively costly hand insertion of coils and more complex field connections.